Method of making cemented carbide insert

ABSTRACT

A method of making a cutting insert includes forming a powder mixture containing WC, 2-10 wt % Co, 4-12 wt % cubic carbides, adding N in an amount of 0.9-1.7% of the weight of the cubic carbides, mixing the powder with a pressing agent, milling and spray drying the mixture, compacting and sintering the material at 1300-1500 in an atmosphere of sintering gas 40-60 mbar, applying post-sintering treatment, and applying a coating by CVD or MTCVD.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a reissue of U.S. Pat. No. 6,699,526 B2,filed on Oct. 11, 2001, which is a divisional application of U.S. Pat.No. 6,333,100, filed on Feb. 2, 2000, which claims the benefit ofpriority of Swedish Application No. 9900403 - 8, filed Feb. 5, 1999.

This application is a divisional of application Ser. No. 09/496,200,filed on Feb. 2, 2000, now U.S. Pat. No. 6,333,100 which issued on Dec.25, 2001.

BACKGROUND OF THE INVENTION

The present invention relates to a coated cutting tool insertparticularly useful for turning of steel, like low alloyed steels,carbon steel and tough hardened steels, at high cutting speeds.

High performance cutting tools must nowadays possess high wearresistance, high toughness properties and good resistance to plasticdeformation. This is particularly so when the cutting operation iscarried out at very high cutting speeds and/or at high feed rates whenlarge amount of heat is generated.

Improved resistance to plastic deformation of a cutting insert can beobtained by decreasing the WC grain size and/or by lowering the overallbinder phase content, but such changes will simultaneously result insignificant loss in the toughness of the insert.

Methods to improve the toughness behaviour by introducing a thickessentially cubic carbide free and binder phase enriched surface zonewith a thickness of about 20-40 μm on the inserts by so called gradientsintering techniques are in the art.

However, these methods produce a rather hard cutting edge due to adepletion of binder phase and enrichment of cubic phases along thecutting edge. A hard cutting edge is more prone to chipping.Nevertheless, such carbide inserts with essentially cubic carbide freeand binder phase enriched surface zones are extensively used today formachining steel and stainless steel.

There are ways to overcome the problem with edge brittleness bycontrolling the carbide composition along the cutting edge by employingspecial sintering techniques or by using certain alloying elements, ofwhich U.S. Pat. No. 5,484,468, U.S. Pat. No. 5,549,980, U.S. Pat. No.5,729,823 and U.S. Pat. No. 5,643,658 are illustrated.

All these techniques give a binder phase enrichment in the outermostregion of the edge. However, inserts produced according to thesetechniques often obtain micro plastic deformation at the outermost partof the cutting edge. In particular, this often occurs when the machiningis carried out at high cutting speeds. A micro plastic deformation ofthe cutting edge will cause a rapid flank wear and hence a shortenedlifetime of the cutting inserts. A further drawback of theabove-mentioned techniques is that they are complex and difficult tofully control.

U.S. Pat. No. 5,786,069 and U.S. Pat. No. 5,863,640 disclose coatedcutting tool inserts with a binder phase enriched surface zone and ahighly W-alloyed binder phase.

SUMMARY

The present invention provide a cutting tool insert for machining steel,including a cemented carbide body and a coating, wherein: the cementedcarbide body includes WC, 2-10 wt. % of Co, 4-12 wt. % of cubic carbidesof metals from groups 4, 5 or 6 of the periodic table, and N in anamount of between 0.9 and 1.7% of the weight of the elements from groups4 and 5; the cemented carbide body includes a Co-binder phase which ishighly alloyed with W, and has a CW-ratio of 0.75-0.90; the cementedcarbide body has a surface zone with a thickness of <20 μm, which isbinder phase enriched and essentially cubic carbide free; the cementedcarbide body has a cutting edge which has a binder phase content whichis 0.65-0.75 of the bulk binder phase content, and the binder phasecontent increases at a constant rate along a line which bisects saidcutting edge, until it reaches the bulk binder phase content at adistance between 100 and 300 μm from the cutting edge; and the coatingincludes a 3-12 μm columnar TiCN layer followed by a 2-12 μm Al₂O₃layer, possibly with an outermost 0.5-4 μm TiN layer.

The present invention also provides a method of making a cutting insertcomprising a cemented carbide body having a binder phase, with a binderphase enriched surface zone and a binder phase depleted cutting edge,and a coating, including the steps of: forming a powder mixtureincluding WC, 2-10 wt. % Co, 4-12 wt. % of cubic carbides of metals fromgroups 4, 5 or 6 of the periodic table, the binder phase having aCW-ratio of 0.75-0.90; adding N in an amount of between 0.9 and 1.7% ofthe weight of the elements from groups 4 and 5; mixing the powder with apressing agent; milling and spray drying the mixture to a powdermaterial compacting and sintering the powder material at a temperatureof 1300-1500° C., in a controlled atmosphere of sintering gas at 40-60mbar followed by cooling; applying post-sintering treatment; andapplying a hard, wear resistant coating by CVD or MT-CVD-technique.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing of a cross section of an edge of an insertgradient sintered according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It has now surprisingly been found that significant improvements withrespect to resistance to plastic deformation and toughness behaviour cansimultaneously be obtained for a cemented carbide insert if a number offeatures are combined. The improvement in cutting performance of thecemented carbide inserts can be obtained if the cobalt binder phase ishighly alloyed with W, if the essentially cubic carbide free and binderphase enriched surface zone A has a certain thickness and composition,fi the cubic carbide composition near the cutting edge B is optimisedand if the insert is coated with a 3-12 μm columnar TiCN-layer followedby a 2-12 μm thick Al₂O₃ layer, for example produced according to any ofthe patents U.S. Pat. No. 5,766,782, U.S. Pat. No. 5,654,035, U.S. Pat.No. 5,674,564 or U.S. Pat. No. 5,702,808, possibly with an outermost0.5-4 μm TiN-layer. The Al₂O₃-layer will serve as an effective thermalbarrier during cutting and thereby improve not only the resistance toplastic deformation which is a heat influenced property but alsoincreases the crater wear resistance of the cemented carbide insert. Inaddition, if the coating along the cutting edge is smoothed by anappropriate technique, like by brushing with a SiC-based nylon brush orby a gentle blasting with Al₂O₃ grains, the cutting performance can beenhanced further, in particular with respect to flaking resistance ofthe coating (see, e.g. U.S. Pat. No. 5,851,210).

Said cutting insert possess excellent cutting performance when machiningsteel at high cutting-speeds, in particular low alloyed steels, carbonsteels and tough hardened steels. As a result a wider application areafor the coated carbide insert is obtained because the cemented carbideinsert according to the invention performs very well at both low andvery high cutting speeds under both continuous and intermittent cuttingconditions.

The coated cemented carbide insert of the invention has a <20 μm,preferably 5-15 μm, thick essentially cubic carbide free and binderphase enriched surface zone A (FIG. 1), preferably with an averagebinder phase content (by volume) of 1.2-3.0 times the bulk binder phasecontent. In order to obtain high resistance to plastic deformation butsimultaneously avoid a brittle cutting edge the chemical composition isoptimised in zone B (FIG. 1). Along line C (FIG. 1), in the directionfrom edge to the centre of the insert, the binder phase contentincreases essentially constantly until it reaches the bulk composition.At the edge the binder phase content by volume is 0.65-0.75, preferablyabout 0.7 times the binder phase content of the bulk. In a similar way,the cubic carbide phase content decreases along line C, preferably fromabout 1.3 times the content of the bulk. The depth of the binder phasedepletion and cubic carbide enrichment along line C is 100-300 μm,preferably 150-250 μm.

The binder phase is highly W-alloyed. The content of W in the binderphase can be expressed as a

CW-ratio=M_(s)/(wt. % Co*0.0161) where M_(s) is the measured saturationmagnetisation of the cemented carbide body in kA/m hAm² /kg and wt-% Cois the weight percentage of Co in the cemented carbide. The CW-ratiotakes a value ≦1 and the lower the CW-ratio, the higher is the W-contentin the binder phase. It has now been found according to the inventionthat an improved cutting performance is achieved if the CW-ratio is0.75-0.90, preferably 0.80-0.85.

Inserts according to the invention are further provided with a coatingconsisting of essentially 3-12 μm columnar TiCN-layer followed by a 2-12μm thick Al₂O₃-layer deposited, for example according to any of thepatients U.S. Pat. No. 5,766,782, U.S. Pat. No. 5,654,035, U.S. Pat. No.5,674,564, U.S. Pat. No. 5,702,808 preferably with an α-Al₂O₃-layer,possibly with an outermost 0.5-4 μm TiN-layer.

The present invention is applicable to cemented carbides with acomposition of 2-10, preferably 4-7, weight percent of binder phaseconsisting of Co, and 4-12, preferably 7-10, weight percent curbiccarbides of the metals from groups 4, 5 or 6 of the periodic table,preferably >1 wt. % of each Ti, Ta and Nb and a balance WC. The WCpreferably has an average grain size of 1.0 to 4.0 μm, more preferably2.0 to 3.0 μm. The cemented carbide body may contain small amounts, <1volume %, of η-phase (M₆C).

By applying layers with different thicknesses on the cemented carbidebody according to the invention, the property of the coated insert canbe optimised to suit specific cutting conditions. In one embodiment, acemented carbide insert produced according to the invention is providedwith a coating of: 6 μm TiCN, 8 μm Al₂O₃ and 2 μm TiN. This coatedinsert is particularly suited for cutting operation with high demandregarding crater wear. In another embodiment, a cemented carbide insertproduced according to invention is provided with a coating of: 8 μmTiCN, 4 μm Al₂O₃ and 2 μm TiN. This coating is particularly suited forcutting operations with high demands on flank wear resistance.

The invention also relates to a method of making cutting insertscomprising a cemented carbide substrate consisting of a binder phase ofCo, WC and a cubic carbonitride phase with a binder phase enrichedsurface zone essentially free of cubic phase and a coating. The powdermixture consists 2-10, preferably 4-7, weight percent of binder phaseconsisting of Co, and 4-12, preferably 7-10, weight percent cubiccarbides of the metals from groups 4, 5 or 6 of the periodic table,preferably >1 wt. % of each Ti, Ta and Nb and a balance WC, preferablywith an average grain size of 1.0-4.0 μm, more preferably 2.0-3.0 μm.Well-controlled amounts of nitrogen are added either through the powderas carbonitrides and/or added during the sintering process via thesintering gas atmosphere. The amount of added nitrogen will determinethe rate of dissolution of the cubic phases during the sintering processand hence determine the overall distribution of the elements in thecemented carbide after solidification. The optimum amount of nitrogen tobe added depends on the composition of the cemented carbide and inparticular on the amount of cubic phases and varies between 0.9 and1.7%, preferably about 1.1-1.4%, of the weight of the elements fromgroups 4 and 5 of the periodic table. The exact conditions depend to acertain extent on the design of the sintering equipment being used. Itis within the purview of the skilled artisan to determine whether therequisite surface zones A and B of cemented carbide have been obtainedand to modify the nitrogen addition and the sintering process inaccordance with the present specification in order to obtain the desiredresult.

The raw materials are mixed with pressing agent and possibly W such thatthe desired CW-ratio of the binder phase is obtained and the mixture ismilled and spray dried to obtain a powder material with the desiredproperties. Next, the powder material is compacted and sintered.Sintering is performed at a temperature of 1300-1500° C., in acontrolled atmosphere of between 40 and 60 mbar, preferably about 50mbar, followed by cooling. After conventional post sintering treatmentsincluding edge rounding a hard, wear resistant coating, such as definedabove, is applied by CVC- or MT-CVD-technique.

EXAMPLE 1

A.) Cemented carbide turning inserts of the style C N M G 1 2 0 4 0 8 -P M, D N M G 1 5 0 6 1 2 - P M and CNMG160616-PR, with the composition5.5 wt. % Co, 3.5 wt. % TaC, 2.3 wt. % NbC, 2.1 wt. % TiC and 0.4 wt. %TiN and balance WC with an average grain size of 2.5 μm were producedaccording to the invention. The nitrogen was added to the carbide powderas TiCN. Sintering was done at 1450° C. in a controlled atmosphereconsisting of Ar, CO and some N₂ at a total pressure of about 50 mbar.

Metallographic investigation showed that the produced cemented carbideinserts had a cubic-carbide-free zone A with a thickness of 10 μm. Imageanalysis technique was used to determine the phase composition at zone Band the area along line C (FIG. 1). The measurements were done onpolished cross sections of the inserts over an area of approx. 40×40 μmgradually moving along the line C. The phase composition was determinedas volume fractions. The analysis showed that the cobalt content in zoneB was 0.7 times the bulk cobalt cement and the cubic carbide content 1.3times the bulk gamma phase content. The measurements of the bulk contentwere also done by image analysis technique. The Co-content was graduallyincreasing and the cubic carbide content gradually decreasing along lineC in the direction from the edge to the centre of the insert.

Magnetic saturation values were recorded and used for calculatingCW-values. An average CW-value of 0.84 was obtained.

B.) Inserts from A were first coated with a thin layer <1 μm of TiNfollowed by 6 μm thick layer of TiCN with columnar grains by usingMT-CVD-techniques (process temperature 850° C. and CH₃CN as thecarbon/nitrogen source). In a subsequent process step during the samecoating cycle, an 8 μm thick α-Al₂O₃ layer was deposited according topatent U.S. Pat. No. 5,654,035. On top of the α-Al₂O₃ layer a 1.5 μm TiNlayer was deposited.

C.) Inserts from A were first coated by a thin layer <1 μm of TiNfollowed by a 9 μm thick TiCN-layer and a 5 μm thick α-Al₂O₃ layer and a2 μthick TiN layer on top. The same coating procedures as given in A.)were used.

D.) Commercially available cutting insert in style C N M G 1 2 0 4 0 8 -P M, D N M G 1 5 0 6 1 2 - P M and CNMG160616-PR, with the compositiongiven below were used as references in the cutting tests:

-   -   Composition: Co=5.5 wt. %, TaC=5.5 wt. %, NbC=2.3 wt. %, TiC=2.6        wt. % and balance WC with a grain size 2.6 μm. Cobalt enriched        gradient zone: none    -   CW-ratio: >0.95    -   Coating: 8 μm TiCN, 6 μm, Al₂O₃, 0.5 μm TiN on top

E.) Inserts with the same cemented carbide composition as in D werecoated with a 4 μm TiN and 6 μm Al₂O₃. Inserts styles CNMG120408-QM andCNMG120412-MR.

F.) Inserts in styles C N M G 1 2 0 4 0 8 - Q M and CNMG120412-MR withthe composition: 4.7 wt. % Co, 3.1 Wt. % TaC, 2.0 wt. % NbC, 3.4 wt. %,TiC 0.2 wt. % N and rest WC with a grain size of 2.5 μm were produced.The inserts were sintered according to the method described in patentU.S. Pat. No. 5,484,468, i.e., a method that gives cobalt enrichment inzone B. The sintered carbide inserts had a 25 μm thick gradient zoneessentially free from cubic carbide. The inserts were coated with thesame coating as in E.

EXAMPLE 2

Inserts from B and C of Example 1 were tested and compared with insertsfrom D with respect to toughness in a longitudinal turning operationwith interrupted cuts.

-   -   Material: Carbon steel SS1312.

Cutting data: Cutting speed = 140 m/min Depth of cut = 2.0 mm Feed =Starting with 0.12 mm and gradually increased by 0.08 mm/min untilbreakage of the edge

-   -   15 edges of each variant were tested    -   Insert style: CNMG120408-PM

Results: mean feed at breakage Inserts B 0.23 mm/rev Inserts C 0.23mm/rev Inserts D 0.18 mm/rev

EXAMPLE 3

Inserts from B, C and D of Example 1 were tested with respect toresistance to plastic deformation in longitudinal turning of alloyedsteel (AISI 4340).

Cutting data: Cutting speed = 160 m/min Feed = 0.7 mm/rev. Depth of cut= 2 mm Time in cut = 0.50 min

The plastic deformation was measured as the edge depression at the noseof the inserts.

Results: Edge depression, μm Insert B 43 Insert C 44 Insert D 75

Examples 2 and 3 show that the inserts B and C according to theinvention exhibit much better plastic deformation resistance incombination with somewhat better toughness behaviour in comparison tothe inserts D according to prior art.

EXAMPLE 4

Inserts from E and F of Example 1 were tested with respect to flank wearresistance in longitudinal turning of ball bearing steel SKF25B.

Cutting data: Cutting speed: 320 m/min Feed: 0.3 mm/rev. Depth of cut: 2mm

Tool life criteria: Flank wear >0.3 mm

Results: Tool life Insert E 8 min Insert F 6 min

Variant F exhibited micro plastic deformation resulting in more rapiddevelopment of the flank wear.

EXAMPLE 5

Inserts from E and F of Example 1 in inserts style CNMG120412-MR weretested at an end-user in machining of a steel casting component.

Cutting data: Cutting speed: 170-180 m/min Feed: 0.18 mm/rev. Depth ofcut: 3 mm

The component had the shape of a ring. The inserts machined twocomponents each and the total time in cut was 13.2 min.

After the test the flank wear of the inserts were measured.

Results: Flank wear Insert E 0.32 mm Insert F 0.60 mm

Example 4 and 5 illustrate the detrimental effect of cobalt enrichmentin the edge area B typical for inserts produced by prior art gradientsintering technique as described in e.g. U.S. Pat. No. 5,484,468.

EXAMPLE 6

Inserts from B and D from Example 1 were tested under the same conditionas in Example 4. Inserts style CNMG120408-PM.

Cutting data: Cutting speed: 320 m/min Feed: 0.3 mm/rev. Depth of cut: 2mm

-   -   Tool life criteria: Flank wear >0.3 mm

Results: Tool life Insert B 8 min Insert D 8 min

EXAMPLE 7

Inserts from B and D of Example 1 were tested at an end user in themachining of cardan shafts in tough hardened steel. Insert styleDNMG150612-PM.

Cutting condition: Cutting speed: 150 m/min Feed: 0.3 mm/rev. Depth ofcut: 3 mm

The inserts machined 50 component each. Afterwards the flank wear of theinserts was measured.

Results: Flank wear Insert B 0.15 mm Insert D 0.30 mm

Examples 6 and 7 illustrate that inserts with an optimised edge zonecomposition according to the invention do not suffer from micro plasticdeformation and hence no rapid flank wear as prior art gradient sinteredinsert F does (see examples 4 and 5).

EXAMPLE 8

In a test performed at an end-user inserts from B, C and D in Example 1in style CNMG160616-PR were run in a longitudinal turning operation inmachining of crankshaft in low alloyed steel.

The inserts were allowed to machine 90 crankshafts and the flank wearwas measured and compared.

Cutting data: Cutting speed: 220 m/min Feed: 0.6 mm/rev. Depth of cut:3-5 mm Total time in cut: 27 min.

The dominating wear mechanism was plastic deformation of the type edgeimpression causing a flank wear.

Results: Flank wear Insert B 0.2 mm Insert C 0.2 mm Insert D 0.6 mm

The example illustrates the superior resistance to plastic deformationof the inserts B and C produced according to the invention compared toprior art inserts D.

1. A method of making a cutting insert comprising a cemented carbidebody having a binder phase, with a binder phase enriched surface zone,and a binder phase depleted cutting edge, and a coating, comprising thesteps of: forming a powder mixture containing WC, 2-10 wt. % Co, 4-12wt. % of cubic carbides of metals from groups 4, 5 or 6 of the periodictable, the binder phase having a CW-ratio of 0.75-0.90; adding N in anamount of between 0.9 and 1.7% of the weight of the elements from groups4 and 5; mixing said powder with a pressing agent; milling and spraydrying the mixture to a powder material; compacting and sintering thepowder material at a temperature of 1300-1500° C., in a controlledatmosphere of sintering gas at 40-60 mbar followed by cooling; applyingpost-sintering treatment; and applying a hard, wear resistant coating byCVD- or MT-CVD-technique.
 2. The method of claim 1, wherein the powdermixture comprises 2-7 wt. % Co.
 3. The method of claim 1, wherein thepowder mixture comprises 7-10 wt. % of cubic carbides of the metals fromgroups 4, 5 or 6 of the periodic table.
 4. The method of claim 1,wherein the powder mixture comprises more than 1 wt. % of each Ti cubiccarbide, Ta cubic carbide and Nb cubic carbide.
 5. The method of claim1, wherein N is added in an amount between 1.1 and 1.4% of the weight ofelements from groups 4 and
 5. 6. The method of claim 1, wherein N isadded to the powder mixture as carbonitride.
 7. The method of claim 1,wherein the N is added during the sintering step as part of thesintering gas atmosphere.
 8. The method of claim 1, wherein thesintering is carried out at about 50 mbar.
 9. The method of claim 1,wherein the hard, wear resistant coating is a 3-12 μm columnar TiCNlayer followed by a 2-12 μm Al₂O₃.
 10. The method of claim 1, wherein Wis added to the powder mixture with the pressing agent, so as to achievethe CW-ratio of 0.75-0.90.
 11. The method of claim 10, wherein W isadded to the powder mixture with the pressing agent, so as to achievethe CW-ratio of 0.80-0.85.
 12. The method of claim 1, wherein theCW-ratio is 0.80-0.85.